1. Field of the Invention
The invention relates to silica sol-gel processing methods.
2. Discussion of the Related Art
Glass optical fiber is typically drawn from a solid preform containing an overcladding that surrounds an inner cladding and core. An overcladding tube is generally formed separately from the inner cladding and core, and the components are then brought together to make the preform. The overcladding does not have to meet the demanding purity and uniformity specifications of the core and inner cladding, and some efforts at lowering the cost of optical fiber manufacturing processes have therefore focused on the overcladding. These efforts have led to the use of sol-gel processes to form overcladding tubes.
U.S. Pat. No. 5,240,488, the disclosure of which is hereby incorporated by reference, discloses a sol-gel process for production of overcladding tubes. In the process, an aqueous colloidal silica dispersion is used. The dispersion is typically stabilized by addition of a base such as tetramethylammonium hydroxide (TMAH). TMAH is believed to stabilize silica particles by the following mechanism: Introduction of the TMAH solution into a silica dispersion raises the pH value. The silica then takes on a negative surface charge due to ionization of silanol groups present on the surface, in accordance with the following reaction: ##STR1## The negative charge on the silica particles creates mutual repulsion, preventing substantial agglomeration and maintaining the stability of the dispersion. A pH of about 11 to 14 is indicated to be required. At a later stage in the process, as discussed at Col. 15, lines 39-65 of the '488 patent, a gelling agent such as methyl formate is added to reduce the pH. The methyl formate, through reaction with water and/or base, neutralizes the negatively-charged silica to a degree where gelation is induced, i.e., reduces the zeta potential to near-zero. (Zeta potential is the potential across the diffuse layer of ions surrounding a charged colloidal particle, and is typically measured from electrophoretic mobilities--the rate at which colloidal particles travel between charged electrodes placed in a solution. A negative or positive zeta potential indicates some repulsion between particles.)
While processes such as that of the '488 patent produce good results, use of TMAH has caused some inconveniences in large-scale optical fiber fabrication. As shown in the Table in Cols. 11 and 12 of the '488 patent, once a gelled tube is dried, the tube must be heated to drive off volatile organic materials and water. During this heating, the tetramethylammonium salt that is present (e.g., tetramethyl ammonium formate in the reaction of the '488 patent) breaks down to an extent into trimethylamine (TRIMA), which has an undesirable odor, and is volatile and flammable. Similarly, methyl formate produces a by-product of formaldehyde, which is also an odorous material.
Thus, the use of TMAH, as well as methyl formate, in sol-gel overcladding fabrication has the potential to complicate an optical fiber manufacturing process. Alternative sol-gel fabrication techniques have therefore been sought.
One attempt at an alternative solution is reflected in Japanese Kokai Patent Application No. 1-183421 (the '421 application). The method of the '421 application involves (1) forming linear polysiloxane polymer by reacting a full alkoxysilane, i.e., a tetraalkoxysilane monomer, in the presence of an acidic catalyst, (2) preparing a silica particle sol by a conventional TEOS (tetraethoxysilane) technique in the presence of a basic catalyst, and (3) mixing the two sols to obtain gelation. According to the examples of the '421 application, however, attaining gelation is difficult and time-consuming. Specifically, in the examples, gelation was reached only by holding the mixed sols at 5.degree. C. for an undisclosed time, and then waiting 2-3 more days before the gelled body could be handled. Drying then took 15-25 days at 60-70.degree. C. and 3 days at 100.degree. C. These conditions are not acceptable for most applications.
A type of hybrid material also using alkoxysilanes is described in F. Babonneau, "Hybrid Siloxane-Oxide Materials Via Sol-Gel Processing: Structural Characterization," Polyhedron, Vol. 13, No. 8, 1123 (1994). Babonneau discusses hybrid material formed by reacting a mixture (the sol) of tetra-functional alkoxysilanes and di- and/or tri-functional alkoxysilanes, to obtain a cross-linked structure (the gel), as reflected in FIG. 8 of the reference. The di- or tri-functional alkoxysilanes appear to act as roadblocks for the propagation of the tetrafunctional alkoxysilane (Si(OR).sub.4), resulting in the hybrid cross-linked structure. However, these systems tend to exhibit small pores that lead to high capillary pressures during drying. The high capillary pressures cause cracking and breakage, such that formation of bodies larger than 1 kg has been extremely difficult. The applicability of such hybrid materials for commercial uses such as overcladding tubes is therefore questionable.
Thus, further improvements and alternatives for sol-gel fabrication are desired.